The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of IC processing and manufacturing. For these advances to be realized, similar developments in IC processing and manufacturing are needed. For example, the need to perform higher resolution lithography processes grows. One lithography technique is extreme ultraviolet lithography (EUVL). Other techniques include X-Ray lithography, ion beam projection lithography, electron beam projection lithography, and multiple electron beam maskless lithography.
The EUVL employs scanners using light in the extreme ultraviolet (EUV) region, having a wavelength of about 1-100 nm. Some EUV scanners provide 4× reduction projection printing, similar to some optical scanners, except that the EUV scanners use reflective rather than refractive optics, i.e., mirrors instead of lenses. EUV scanners provide the desired pattern on an absorption layer (“EUV” mask absorber) formed on a reflective mask. Currently, binary intensity masks (BIM) are employed in EUVL for fabricating integrated circuits. EUVL is similar to optical lithography in that it needs a mask to print wafers, except that it employs light in the EUV region, i.e., at 13.5 nm. At the wavelength of 13.5 nm or so, all materials are highly absorbing. Thus, reflective optics rather than refractive optics is used. A multilayered (ML) structure is used as a EUV mask blank.
However, conventional EVU masks and the fabrication thereof may still have drawbacks. For example, the ML structure in conventional EUV masks is a periodic structure, which may include 40 pairs of silicon and molybdenum. Each pair of silicon and molybdenum has the same thickness as the rest of the pairs. The periodic ML structure may lead to low reflectivity and diffraction imbalance, which are undesirable.
Therefore, while conventional EUV lithography systems and processes have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect. What is needed is a EUV lithography method and system having improved reflectivity and diffraction balance.